How should I use assertions in Launchpad code?

What exceptions am I allowed to catch?

What exception should I raise when something passed into an API isn't quite right?

In short, never raise ValueError, NameError or TypeError, and avoid subclassing these exceptions as well. The full instructions are at ExceptionGuidelines.

In the case of NotFoundError, if you are going to catch this specific error in some other code, and then take some corrective action or some logging action, then seriously consider creating your own subclass. This allows your code to handle exactly the situation that you expect, and not be tricked into handling NotFoundErrors raised by code several levels in.

When writing docstrings, always think whether it makes things clearer to say which exceptions will be raised due to inappropriate values being passed in.

I have a self-posting form which doesn't display the updated values after it's submitted. What's wrong?

For now, all self-posting forms have to call canonical.database.sqlbase.flush_database_updates() after processing the form.

SQL Result Set Ordering

If the ordering of an SQL result set is not fully constrained, then your tests should not be dependent on the natural ordering of results in the sample data.

If Launchpad does not depend on the ordering of a particular result set, then that result set should be sorted within the test so that it will pass for any ordering.

As a general rule, the result sets whose order we want to test are the ones that are displayed to the user. These should use an ordering that makes sense to the user, and the tests should ensure that happens.

For code that uses SQLObject, result sets will be randomised while conforming to the specified ORDER BY clause. This makes it difficult to write tests that depend on the natural ordering.

In contrast, code that does raw SQL won't have a randomised result set, so the natural order gets exposed to the user. The same rules of thumb apply when testing such code, but extra care should be taken not to rely on natural ordering.

How do I use a postgres stored-procedure/expression as the orderBy in SQLObject?

If you need to embed your data in the SQL query itself, there is only one rule you need to remember - quote your data. Failing to do this opens up the system to an SQL Injection attack, one of the more common and widely known security holes and also one of the more destructive. Don't attempt to write your own quote method - you will probably get it wrong and end up with the DBA removing precious parts of your anatomy. The only two formats you can use are %d and %s, and %s should always be escaped, *no exceptions!*

(the second command in the previous example demonstrates a simple argument that might be passed in by an attacker).

What date and time related types should I use?

See DatetimeUsageGuide for information on what types to use, and how the Python datetime types relate to database types through SQLObject.

Python segfaulted. What should I do?

Python programs should never segfault, but it can happen if you trigger a bug in an extension module or the Python core itself. Since a segfault won't give you a Python traceback, it can be a bit daunting trying to debug these sort of problems. If you run into this sort of problem, tell the list.

See DebuggingWithGdb for some tips on how to narrow down where a segfault bug like this is occurring -- in some cases you can even get a Python stack trace for this sort of problem.

I want an object to support __getitem__, what's the best style?

Many Launchpad objects support __getitem__. For example, if you have a Foo, and want to support Foo()['bar'], you will implement __getitem__ for class Foo. Often, this is used along with GetitemNavigation in your browser code to ensure smooth traversal.

The __getitem__ code itself should not, however, contain the magic that fetches whatever needs to be fetched. It should instead call another method that does so, explicitly saying what it is getting. So for example:

Note that generally, a __getitem__ method should give access to just one kind of thing. In the example above, it gives you access to versions with the given name. If your traversal needs to get two kinds of things, for example versions or changesets, then this is better put in the traversal code in the FooNavigation class than in the __getitem__ code of the database class.

I believe that the Launchpad coding style has evolved to frown upon unnecessary use of __magic__ methods. They make the code harder to grep and refactor. __getitem__ should only be used exceptionally when an object is meant to implement protocols that rely on it (list, dictionnary, string). Maybe this section should be updated to read something like "Don't". -- DavidAllouche 2007-11-14 16:43:09

Properties

Properties should be cheap. Using a property can make accessing fields or calculated results easier, but programmers expect properties to be usable without consideration of the internal code in the property. As such, a property that calls expensive routines such as disk resources, examining database joins or the like will violate this expectation. This can lead to hard to analyse performance problems because its not clear what is going on unless you are very familiar with the code

Our code routinely contradicts this guideline. I remember I had issues in the past with TALES traversal when trying to use methods, and had to use properties instead. We have decorators such as @cachedproperty to help with the performance issues. Someone who knows what he talks about should update this FAQ to match reality. -- DavidAllouche 2007-11-14 16:50:06

Properties should always be used instead of __call__() semantics in TALES expressions. The rule is that in view classes, we don't do this:

def foo(self):
...

We always do this:

@property
def foo(self):
...

Storm

How to retrieve a store ?

There are two ways of retrieving a storm 'store', before issuing a query using native syntax.

The first format retrieves the Store being used by another object. Use this method when you don't need to make changes, but want your objects to interact nicely with objects from an unknown Store (such as a methods parameters):

You can also explicitly specify what Store you want to use. You get to choose the realm (Launchpad main, auth database) and the flavor (master or slave). If you are retrieving objects that will need to be updated, you need to use the master. If you are doing a search and we don't mind querying data a few seconds out of date, you should use the slave.

If you don't need to update, but require up-to-date data, you should use the default flavor. (eg. most views - the object you are viewing might just have been created). This will retrieve the master unless the load balancer is sure all changes made by the current client have propagated to the replica databases.

Security, authentication

How can I do get the current user in a database class?

You need to pass it in one of the parameter's method. (You shouldn't use the ILaunchBag for this. In fact, you shouldn't use the ILaunchBag in any database class.) The principle is that the database code must not rely on implicit state, and by that is meant state not present in the database object's data nor

in the arguments passed to the method call. Using ILaunchBag or check_permission would use this kind of implicit state.

How can I protect a method based on one of its parameter?

You can't! Only attribute access can be protected and only the attribute name and the current user is available when that check is made. But there is a common pattern you can use: call in that method another method on the object passed as parameter. That method can be appropriately protected using the current security infrastructure. Since this auxillary method

is part of an object-collaboration scenario, it's usually a good idea to start these methods with the notify verb. The method is notifying the other object that a collaboration is taking place. This will often happen with methods that needs to operate on bugs (since you usually don't want the operation to be allowed if it's a private bug that the user doesn't have access to). Example:

def linkBug(self, bug):
# If this is a private bug that the user doesn't have access, it
# will raise an Unauthorized error.
bug.notifyLinkBug(self)

Email Notifications

When I need to send a notification for a person/team, how do I know what email address(es) I have to send the notification to?

As you know, persons and teams are meant to be interchangeable in Launchpad, but when it comes to mail notification the rules differ a bit (see TeamEmail for more information). In order to mask these rules, there's a helper function called contactEmailAddresses() in lib/canonical/launchpad/helpers.py that you should always use to get the contact address of a given person/team. (please note that this function will always return a Set of email addresses, which is perfectly suitable to be passed in to simple_sendmail())

Web UI

How do I perform an action after an autogenerated edit form has been successfully submitted?

You need to write a view's class for this form, if you don't have one already. In your view's class, add

a method changed().

1defchanged(self): 2# This method is called after changes have been made.

You can use this hook to add a redirect, or to execute some logging, for example.

How do I perform an action after an autogenerated add form has been successfully submitted?

You need to write a view's class for this form, if you don't have one already. In your view's class, add

a method createAndAdd().

1defcreateAndAdd(self, data): 2# This method is called with the data from the form.

You can use this hook to create new objects based on the input from the user.

How can I redirect a user to a new object just created from an autogenerated add form?

You need to write a view's class for this form, if you don't have one already. In your view's class, add

a method nextURL().

1defnextURL(self): 2# This method returns the URL where the user should be redirected.

How do I format dates and times in page templates?

Let's use some object's datecreated attribute as an example.

To format a date, use tal:content="context/datecreated/fmt:date

To format a time, use tal:content="context/datecreated/fmt:time

To format a date and time, use tal:content="context/datecreated/fmt:datetime

How should I generate notices like "Added Bug #1234" to the top of the page?

How can I temporarily override configuration variables in tests?

Testing

What kind of tests should we use?

Short answer is that we favor the use of doctest in lib/canonical/launchpad/doc for API documentation and PageTests for use-cases documentation. We use doctests and regular python unittest to complete the coverage.

How do I run just one doctest file, e.g. `lib/canonical/launchpad/doc/mytest.txt`?

Use the --test argument to name it:

bin/test -f --test=mytest.txt

What about running just one pagetest story, e.g. `lib/canonical/launchpad/pagetests/initial-bug-contacts`?

bin/test -f canonical pagetests.initial-bug-contacts

What about running a standalone pagetest, e.g. `xx-bug-index.txt`

Like this:

bin/test -f canonical xx-bug-index

Note that you don't include the .txt.

And if I want to execute all test except one?

bin/test '!test_to_ignore'

How can I examine my test output with PDB?

bin/test's -D argument is everyone's friend. If your test raises any exceptions or failures, then the following will open a pdb shell right where the failure occurred:

bin/test -D -vvt my.test.name

Where can I get help on running tests?

Try this:

bin/test --help

How can I check test coverage?

The bin/test script has a --coverage option that will report on code coverage.

How can I run only the tests for the page test layer?

bin/test --layer=PageTestLayer

Where should I put my tests: in a `test_foo.py` module, or a `foo.txt` doctest file?

You should prefer doctests. A good rule of thumb is that test_*.py modules are best for tests that aren't useful for documentation, but instead for increasing test coverage to obscure or hard-to-reach code paths. It is very easy to write test code that says "check foo does bar", without explaining why. Doctests tend to trick the author into explaining why.

However! Resist the temptation to insert tests into the system doctests (lib/canonical/launchpad/doc/*.txt) that reduce their usefullness as documentation. Tests which confuse rather than clarify do not belong here. To a lesser extent, this also applies to other doctests too.

How to I setup my tests namespace so I can remove unwanted import statements and other noise?

For DocFileSuite tests, such as the system documentation tests, you can pass in setUp and tearDown methods. You can stuff values into the namespace using the setUp method. See canonical/launchpad/ftests/test_system_documentation.py for examples, or the Python Reference guide.

Why is my page test failing mysteriously?

This is often due to a bug in the doctest code that means that ellipses ("...") don't match blank lines ("<BLANKLINE>"). Inserting blank lines in the right parts of the page test should fix it. If you are running a single test and getting odd database failures, changes are you haven't run make schema. When running a single test the database setup step is skipped, and you need to make sure you've done it before.

I'm writing a pagetest in the standalone directory that changes some objects. Will my changes be visible to other pagetests in this directory?

Because by default, the page test ignore them so you don't need to take care of the indentation. Sometimes, the indentation matters (inside <pre> and <textarea> tags) and if you want to test those, you will need to append to the test '#doctest: -NORMALIZE_WHITESPACE', for instance:

>>> print line #doctest: -NORMALIZE_WHITESPACEfoo

What does the number in square brackets after a doctest name mean?

When you get a warning or an error from a doctest, or you see it in a traceback, you'll see the name written with a number in square brackets, like this:

<doctest gpg-encryption.txt[16]>

The number, in the text above it is "16", is the index of the "doctest example" in that file. What this means is that the first >>> in the file is example zero. The next >>> is example one, and so on. So [16] refers to the seventeenth line starting with >>> in the doctest file gpg-encrpytion.txt.

How do I find a backtrace?

Backtraces are kept on chinstrap in the /srv/gangotri-logs directory.

Should I rely on the order of os.listdir?

No. The order in which results are returned by os.listdir is not defined. Wrap your calls with sorted() if the order is important to passing the test.

How do I find the tests that cover the code I've changed?

Although you've written the tests first before changing code, a lot of code is also exercised by many other tests and it's not immediately obvious which those might be without running the whole test suite. There is an experimental tool to help with this at TestsFromChanges.

Sample Data

Sampledata should be innocuous, such that if it is viewed by people outside Canonical, it will not be embarrassing or reveal company-confidential information.

Where can I find a list of the sample users?

As an aid to testing, the sample database provides a set of sample users with differing memberships and authorization levels. Take a look at "lib/canonical/launchpad/pagetests/README.txt" for more information.

How to I make changes to the sample Launchpad database?

You shouldn't make changes to sample data unless you are submitting your changes to db-devel. You can save the state of the development site's database and replace the current-dev.sql so that everyone can see a working site.

make schema to create a clean db.

Do one of these next two.

If you have a patch (say, database/schema/patch-2208-99-0.sql), apply the patch to launchpad_ftest_playground and launchpad_dev.

Note that as a side effect of the sampledata being automatically generated, you will often get difficult to resolve conflicts if you have modified the sample data and attempt to merge in another branch that has also modified it. To work around this, it is recommended that your sampledata changes are always maintained as list of statements in a .sql file so that you can easily reset your current.sql to the launchpad trunk and replay your changes against it (psql -d launchpad_dev -f mysampledatachanges.sql).

I've only made minor changes to the sample data, but the diff is huge!

This often happens when you have modified the schema. Don't worry - if the tests pass, then the modified sample data is good.

Code Reviews

How do I use Meld with Bazaar to do code reviews?

Simple way is to use the Bazaar difftools plugin, which will compare a Launchpad branch against the most recent version of launchpad the branch has

compare branches. The basic form is bzr diff --using=meld branch1 branch2. The branches do not have to be on your local machine. If you try to compare remote branches difftools will take care of putting a working tree into a temp directory for meld to use, and then cleaning up the tree when you close the diff tool. Of course, if you compare local branches it is a bit faster and you can edit things in-place from inside of meld.

You can define an alias in bazaar instead of typing diff --using=meld all the time. Just add something like this to your ~/.bazaar/bazaar.conf:

[ALIASES]
mdiff = diff --using=meld

Bazaar

How do I revert some revisions from my archive?

If they are the most recent commits:

bzr uncommitIf they are not the most recent commits, then you need to do a reverse merge:

bzr merge -r newer..olderfollowed by a commit: bzr commit

I merged from another branch and did a lot of changes without first committing the merge. How can I undo only the merge?

If you are sure your changes don't touch the same code as the merge does, you can do this and it'll probably work:

this should be updated - in bzr we can do a merge into a copy of the branch, then apply that in reverse mode.

On the other hand, if your changes touch the same code as the merge, it's better to do the following

still needs an update to bzr Assuming the worktree you have the merge and your changes is ~/wtree1, the tree-version of that worktree is me@canonical.com/cat--branch--ver and the branch you merged from is remote@host/cat--branch--ver, you have to do this:

How can I get error failures from PQM by email?

If you request a merge and it fails, PQM sends you back an email to the user address that requested the merge. If that machine is reachable from outside your network and is able to handle mail, you don't need to do anything. If you are behind a firewall or without an static IP address in your machine, you will need to masq those emails with a valid email address:

Create or edit the file /etc/postfix/canonical and add a line like:

#UNIX_USER EMAIL_ADDRESS
carlos carlos.perello@canonical.com

Where UNIX_USER is the username you use in your system and EMAIL_ADDRESS is the email where you want to get all PQM mails.

Now, update your system config to use that new file, edit /etc/postfix/main.cf and add a line like:

canonical_maps = hash:/etc/postfix/canonical

Finally, you will need to create a file that postfix is able to understand executing the following command, being inside the postfix directory:

carlos@Gollum:/etc/postfix$postmap canonical

Note, that this change will apply to all emails sent from that account not only to PQM ones, the From: header will point to the specified email address.

How do I set up connection multiplexing for my SSH session?

Rollouts

When will my code changes end up on production?

PQM is frozen sometime on, or shortly after, Friday of week 3. Authorized Release Critical changes are then processed until Wednesday afternoon when the roll-out usually happens.

Bug fixes discovered in this release on the staging server may be cherry picked into the release. Further discussion can be found on the StagingServer page.

I have an urgent bug fix

Get it reviewed and committed to rocketfuel as normal. If it's during Week 4, contact the release manager (usually kiko). Otherwise, follow the PolicyandProcess/EChangePolicy

What is the staging server?

The staging server is rebuilt daily using a copy of the production database and the HEAD of launchpad. It lets you see if your code changes actually work on the production system and performance is good. It lives at https://staging.launchpad.net/.

How do I run scripts with the Python environment of Launchpad?

Use bin/py in your trunk, or whatever other branch you want to use. You might also want to use bin/ipy, if you like IPython.

Buildout

We use zc.buildout for our build system.

Where can I read about it?

You can read about how we set it up in doc/buildout.txt in your trunk. This includes instructions for adding and upgrading distributions.

You can read more general information at the buildout site, buildout.org.

How can I find out what we are using via buildout?

For direct dependencies, look in setup.py (in the top directory of Launchpad). If it is in the "install_requires" argument, then we are getting it from buildout.

For direct and indirect dependencies, look in bin/run (many scripts will do, but this one is how we start Launchpad). The sys.path will show you everything that we get from eggs (that is, via buildout).

How can I find out what we are using via sourcecode?

The canonical answer is to recursively find all symlinks in lib/, bzrplugins/, and optionalbzrplugins/. Here's a find incantation that will list the results: find lib bzrplugins optionalbzrplugins -lname '*/sourcecode/*' -ls .

You should also be able to look at utilities/sourcedeps.conf. This controls what is updated when sources are updated (for instance, via rocketfuel-get). However, it is maintained manually, so it could be behind the times if someone makes a mistake.

You do NOT answer this question by looking in the sourcecode directory. The sourcecode directory is typically a shared resource for all your branches, so it does not necessarily reflect what the current branch is using. It is also not cleaned out by any of our scripts.

Is the ultimate goal to completely get rid of sourcecode, so that all packages come from buildout?

That was our original goal. We still want to shrink it to a very small size, maybe to exclusively hold canonical-identify-provider and shipit. We then might connect those in using develop eggs, rather than symlinks in lib/.

How do we make custom distributions?

Working with our open-source components (lazr.*)

Where to go for other help

If you have encountered a problem, maybe someone else has and has already documented it on the SolutionsLog. Go have a look! If have a problem and you find a solution, document it there!

Unanswered questions

I have a view class that creates a Foo object, called FooAddView, and the view's context is a Bar object. Should I place the view together with other Foo-related views (in browser/foo.py) and the zcml declaration together with the views that share the same context (in zcml/bar.zcml)?